Anatomy of an Egg
The yolk contains all of the egg's fat and half of its protein. A fresh yolk will be firm and makes a high dome when an egg is broken, and acts as an emulsifier for ingredients that don't normally mix well, like oil and water. The color of the yolk -- anywhere from pale to bright yellow -- depends on what the hen was fed.
The egg white is made up of 87% water and has no fat. The egg white directly surrounding the yolk is called the thick white, and is firmer and thicker than the outer layer. As the egg ages, the entire white becomes more uniform and watery. A cloudy egg white -- often found in very fresh egg --indicates the presence of harmless carbon dioxide, which escapes as the egg ages.
These small, white, stringy pieces help anchor the yolk to the thick white, and are most apparent in fresh eggs. They are harmless, but can be removed through straining for aesthetic purposes.
The pocket of air at the wide end of the egg between the albumen and the shell, which increases as the egg ages.
To test a raw egg's freshness, place the egg in a bowl of water. If it sinks, it's fresh; if it floats, it is filled with too much air and has gone bad. Also, discard any eggs with any blood spots; these indicate that the hen that produced them was sick.
Fresh eggs and older eggs (over 10 days old) do have their uses. Fresher eggs are best for poaching and frying, because they hold their shape well. But when baking puffy meringues and souffles, older eggs are better to use because their whites whip up with more volume. Older eggs that have been hard-boiled are also easier to peel than their fresher counterparts.
The grading of eggs (AA, A, or B by the USDA) is determined by the age, shape, and appearance of the shell, yolk, albumen, and the size and location of the air cell. Grade AA eggs have a firm yolk, a thick albumen, and a small air cell; they will generally be no more than 10 days old. Grade A will be 10 days to 2 weeks old and have a good proportion of thick white to thin. Both AA and A eggs must be oval-shaped. Misshapen eggs automatically receive the grade of B, and are rarely sold in grocery stores; most often they go straight to the manufacturers of egg products.
To see inside the egg without breaking it, egg graders use a technique called candling, which refers to a time when eggs were held up to a candle to see inside. Today, eggs pass on rollers over high-intensity lights to make their interiors visible.
It is best to store the eggs in the carton they come in, rather than transferring them to the plastic egg holders often found on refrigerator doors, where they will be subject to frequent temperature fluctuations as the door opens and closes.
Do You Know?
Contrary to popular belief, brown eggs are not more nutritious than white ones, but they are more expensive because the breed of hens that lay them are larger and require more food.